Channel catfish farming is the largest contributor to U.S. aquacultural production. In 2015, catfish growers in the US produced more than $361 million worth of catfish, of which $201 million (55.7%) was from Mississippi (www.nass.usda.gov). Catfish farmers reported 37% mortality rate (U.S. Department of Agriculture, 2011) due to enteric septicemia of catfish (ESC). Edwardsiella ictaluri (also referred to herein as E. ictaluri) is the causative agent of enteric septicemia of catfish. Like some other species in the Enterobacteriaceae, E. ictaluri has the ability to resist killing by professional phagocytes. In particular, E. ictaluri is resistant to channel catfish neutrophils, which is an important aspect of pathogenesis because (1) neutrophils are the predominant cell type in channel catfish intestinal tract immune cells, and (2) the intestine is an important site of entry for E. ictaluri. E. ictaluri is also resistant to killing by the alternative complement pathway in channel catfish. Although a commercial live attenuated vaccine is now available and antibiotics are still used against E. ictaluri and ESC, ESC is still a major threat to the catfish industry (Klesius and Shoemaker, 1999). Live attenuated vaccines often offer the best prospect for vaccines by providing good protection against diseases through stimulating cellular immune responses (Id.), but the currently available commercial vaccine leaves much to be desired in performance and safety.
The tri-carboxylic acid (TCA) cycle supplies intermediates and ATP for bacterial biosynthesis, thus transcription of genes encoding the enzymes of the TCA cycle is regulated by the availability of amino acids (Somerville et al., 2003). The TCA cycle exerts a metabolic regulation over the synthesis of capsular polysaccharide by gluconeogenesis (Sadykov et al., 2010). Previous research into the role of the metabolic pathway in pathogenic bacteria, such as in Escherichia coli, has demonstrated that succinate dehydrogenase and fumarate reductase as part of the TCA cycle are related membrane-bound enzyme complexes (Cecchini et al., 2002). It has been shown that Salmonella enterica serovar Typhimurium requires glycolysis and glucose for intracellular replication in macrophage, and the complete TCA cycle is needed for full virulence in the murine infection model (Yimga et al., 2006).
In E. ictaluri, some live attenuated vaccine candidates have been developed, including auxotrophic (Lawrence et al., 1997; Thune et al., 1999) and iron-siderophore uptake (Abdelhamed et al., 2013) mutants. Also, we reported that TCA and one-carbon (C1) metabolism pathways contribute to E. ictaluri pathogenesis (Dahal et al., 2014; Dahal et al., 2013). In particular, some TCA and C1 metabolism mutants provided good protection against wild-type E. ictaluri infections in fingerling, which was not the case when tested in catfish fry (14-day old). Thus, there is still a need for a live attenuated bacterial vaccine against ESC in fish, especially catfish, which has improved safety and protection levels of the TCA and C1 metabolism mutants.